EP0356802A2 - Electrochemical process for the production of chromic acid - Google Patents

Abstract

The present invention relates to a process for the production of chromic acid CrO3, of high purity on the basis of the electrochemical principle, the measures to be taken comprising the process steps: 1. preparation and purification of an aqueous sodium chromate/sodium dichromate solution, 2. conversion of said sodium chromate/sodium dichromate solution into a sodium dichromate/chromic acid solution, 3. crystallisation of chromic acid from said sodium chromate/chromic acid solution by evaporation, 4. separation of the crystallised chromic acid from the mother liquor, 5. recirculation of the separated mother liquor in the centrifuge.

Description

• 1. Vorbereitung und Reinigung einer wäßrigen Natrium­chromat/Natriumdichromat-Lösung
• 2. Umwandlung besagter Natriumchromat/Natriumdichro­mat-Lösung in eine Natriumdichromat/Chromsäure-Lö­sung mit einem molaren Verhältnis von Natriumionen zu Chromsäure von 0,45:0,55 bis 0,30:0,70 vermit­tels einer mehrstufigen Membranelektrolyse
• 3. Kristallisation von fester Chromsäure aus dieser Natriumdichromat/Chromsäure-Lösung durch Eindamp­fung auf einen Wassergehalt von ca. 9 bis 20 Gew.-%, vorzugsweise von 12 bis 15 Gew.-% H₂O bei Temperaturen zwischen 55°C und 110°C
• 4. Abtrennen der auskristallisierten Chromsäure von der Mutterlauge durch Zentrifugieren und Auswaschen der anhaftenden Mutterlauge mit einer mindestens 55°C heißen, annähernd gesättigten Chromsäure-Lö­sung und Abzentrifugieren der Waschlösung
• 5. Rezirkulation der in der Zentrifuge abgetrennten Mutterlauge in eine mittlere Stufe der unter 2. aufgeführten mehrstufigen Membranelektrolyse bei gleichzeitiger Ausschleusung eines minderen Anteils der Mutterlauge zwecks Entfernung von Verunreini­gungen aus dem Elektrolyse-Kristallisations-Kreis­lauf

umfassen.The present invention relates to a process for the preparation of chromic acid CrO₃ high purity according to the electrochemical principle, the measures to be taken the process stages

• 1. Preparation and cleaning of an aqueous sodium chromate / sodium dichromate solution
• 2. Conversion of said sodium chromate / sodium dichromate solution into a sodium dichromate / chromic acid solution with a molar ratio of sodium ions to chromic acid of 0.45: 0.55 to 0.30: 0.70 by means of a multi-stage membrane electrolysis
• 3. Crystallization of solid chromic acid from this sodium dichromate / chromic acid solution by evaporation to a water content of about 9 to 20 wt .-%, preferably from 12 to 15 wt .-% H₂O at temperatures between 55 ° C and 110 ° C.
• 4. Separate the crystallized chromic acid from the mother liquor by centrifuging and washing out the adhering mother liquor with an at least 55 ° C hot, approximately saturated chromic acid solution and centrifuging off the washing solution
• 5. Recirculation of the mother liquor separated off in the centrifuge into a middle stage of the multi-stage membrane electrolysis listed under 2. with simultaneous removal of a smaller proportion of the mother liquor in order to remove impurities from the electrolysis-crystallization cycle

include.

Chromic acid CrO₃ is technically produced using three different processes:

In the so-called melting process, sodium dichromate crystals are reacted with concentrated sulfuric acid in a molar ratio of approximately 1: 2 at temperatures of approx. 200 ° C. In the so-called wet process, sulfuric acid and sodium dichromate are brought together in a concentrated aqueous solution. Common to both processes is the inevitable occurrence of chromium-contaminated sodium bisulfate as a melt or as an aqueous solution.

The third method, the membrane electrode, avoids this disadvantage and the associated losses of chromium trolysis of sodium dichromate in aqueous solution. The electrochemical process, as described, for example, in Canadian patent specification A-739 447, is based on the principle that is common to membrane electrolysis with a cation-selective membrane, namely the migration of the cations located in an anode chamber through the cation-selective membrane forming the partition wall to the cathode chamber into the cathode chamber under the influence of the electric field.

Embodiments of the electrochemical process for the production of chromic acid are described in the patent specification CA-A-739 447. From a sodium dichromate solution introduced into an anode compartment, the sodium ions migrate in an electric field through the membrane into the cathode compartment filled with water or aqueous solution and form the hydroxide ions formed on the cathode under hydrogen evolution, an aqueous solution containing sodium ions, while in the anode chamber the remaining dichromate ions are electrically neutralized by the hydrogen cations formed on the anode with simultaneous oxygen evolution.

Overall, this process amounts to a substitution of the sodium ions in the sodium dichromate with hydrogen ions, that is to say the formation of chromic acid. In the course of the conversion of the sodium dichromate solution into an increasing amount of sodium dichromate solution containing chromic acid, the migration of the sodium ions through the Membrane increasingly accompanied by the migration of the hydrogen ions formed in the anode compartment, so that the use of electrical current for the desired process of sodium removal from the anode part, also called current efficiency, is steadily decreasing. This means that a complete conversion of the sodium dichromate into chromic acid is not possible in the anode compartment and only an average degree of conversion is controlled for economic reasons. The chromic acid must then be separated from these solutions by fractional crystallization. A mother liquor remains which contains the sodium dichromate which has not yet been converted electrochemically and residues of chromic acid which have not crystallized out. This solution is usefully used again for further conversion into chromic acid in the electrolysis process. The following problems result from these process principles: On the one hand, the mother liquor adhering to the chromic acid crystals and consisting of almost concentrated sodium dichromate solution must be carefully washed off in order to obtain a pure product; on the other hand, all impurities which are introduced with the sodium dichromate solution are accumulated in the system and ultimately discharged with and in the chromic acid crystals, since only the electrolysis gases hydrogen and oxygen leave the process and the membrane separating the anode compartment for anions and also for polyvalent cations is largely impermeable. It is therefore not possible to obtain a very pure chromic acid using this process. About that In addition, cationic impurities in the sodium dichromate solution used, in particular polyvalent cations, lead to an early exhaustion and destruction of the membrane separating the anode and cathode spaces, probably because it occurs in the range of large pH changes that take place within the membrane in very thin layers Precipitation of insoluble hydroxides and salts of these cations comes.

DE-A-3 020 261 describes a process for the electrochemical production of chromic acid from dichromate, the aim of which is to operate the production of chromic acid with high current efficiency and to remove the impurities introduced with the dichromate.

The process of DE-A-3 020 261 is essentially characterized by the use of a three-room cell, the dichromate solution entering the middle room and exiting from there, poor in dichromate, and sodium ions on the cathode chamber separated by a cation-selective membrane during the flow and Gives off dichromations to the anode space separated by a diaphragm or an anion-selective membrane. This creates a chromic acid solution that is largely free of impurities; however, because of the large electrode spacings forced by the central space, an increased voltage is required for the electrolysis process and therefore, and because of the complicated and fragile three-room structure, this method cannot satisfy.

DE-A-3 020 260 describes the purification of sodium chromate solution for the electrochemical production of chromic acid by subjecting the sodium chromate solution to electrolysis in the anode compartment of a two-room cell with a cation-selective partition and the cationic impurities in the membrane precipitate with simultaneous formation of sodium dichromate in the anode compartment and of an alkaline solution containing sodium ions in the cathode compartment, as is known per se from US Pat. No. 3,305,463. The sodium chromate / sodium dichromate solution thus purified is converted electrochemically into chromic acid in the aforementioned manner. Two significant disadvantages, namely frequent replacement or cleaning of the very expensive membrane loaded with the contaminated cations as well as the necessary conversion of the sodium chromate used into sodium dichromate exclusively with electric current instead of with the significantly cheaper inorganic acids sulfuric acid or carbon dioxide make the proposed procedure economically unattractive.

Accordingly, the object of the present invention is to provide, while preserving the advantages of the electrochemical process for the production of chromic acid, a process with which a very pure, crystalline chromic acid is produced under economic conditions.

The present invention thus relates to a process for the preparation of chromic acid by multi-stage electrolysis of dichromate and / or monochromate solutions in two-room electrolysis cells, their anode and catho which are separated by cation exchange membranes, at temperatures of 50 to 90 ° C, the dichromate and / or monochromate solutions being obtained by digestion of chromium ores and leaching, characterized in that the monochromate solution obtained after leaching, if appropriate after the removal of aluminum , Vanadium and other impurities at 20 to 110 ° C to a pH of 8 to 12 by adding and / or in situ production of carbonate in an amount of 0.01 to 0.18 mol / l (at 300 to 500 g / l Na₂CrO₄), separates the precipitated carbonates or hydroxides, concentrates the solution to a content of 750 to 1000 g / l Na₂CrO₄, converts with CO₂ under pressure into a dichromate-containing solution which introduces dichromate-containing solution into the anode compartment of the electrolytic cell, electrolytically produces a solution containing chromic acid in which the molar ratio of Na ions to chromic acid is 0.45: 0.55 to 0.30: 0.70 and the chromium acid worked up by crystallization, washing and drying.

• 1. Durch Auslagen des unter Verwendung schwefelarmer Brennstoffe hergestellten und den Chromerz-Auf­schlußofen verlassenden Ofenklinkers mit Wasser und Einstellen des pH-Wertes auf 7 bis 9,5 mit Dichro­mat-Lösung und/oder einer anderen mineralischen Säure wird eine Natriumchromat-Lösung mit einem Na­triumchromat-Gehalt von ca. 300 bis 500 g/l er­zeugt, die gegebenenfalls durch Fällung bei pH 10 bis 13 in bekannter Weise von mitgelöstem Vanadat befreit wird.
• 2. Diese Lösung wird sodann bei 20°C bis 110°C, vor­zugsweise 60 bis 90°C, auf pH-Werte vn 8 bis 12, vorzugsweise 9 bis 11, durch Zufuhr von Natronlauge und Kohlendioxid oder aber von Natriumcarbonat in einer Menge entsprechend ca. 0,03 bis 0,1 Mol/l Carbonat eingestellt, um die mehrwertigen Kationen als schwerlösliche Carbonate und/oder Hydroxide auszufällen, wobei die Ausfällung auch mehrstufig bei jeweils zunehmenden Gehalten an Natriumchromat durchgeführt werden kann. Dabei wird eine von mehr­wertigen Kationen bis auf zusammengenommen weniger als 5 mg/l befreite Natriumchromatlösung erhalten.
• 3. In der unter 2. erzeugten Lösung wird anschließend gewünschtenfalls durch sogenannten Selektiv-Katio­nenaustauscher der Gehalt an mehrwertigen Kationen noch weiter herabgesetzt.
• 4. Die nach Schritt 2. und gegebenenfalls 3. erzeugte Lösung wird danach durch ein- oder mehrstufige Ver­dampfung auf Gehalte von 750 bis 1000 g/l an Na₂CrO₄ eingeengt.
• 5. In dieser konzentrierten Lösung wird durch einstu­fige oder mehrstufige Zufuhr von Kohlendioxid bis zu einem Enddruck von 4 bis 15 bar bei einer End­temperatur nicht über 50°C ein pH-Wert unter 6,5 eingestellt und auf diese Weise unter Ausfällung von Natriumbicarbonat eine mindestens 80 %ige Um­wandlung des Natriumchromats in Natriumdichromat erreicht.
• 6. Aus dieser Suspension wird entweder unter weiterbe­stehendem Kohlendioxid-Druck oder aber nach dem Entspannen das Natriumbicarbonat abgetrennt, wobei bei letzterem Verfahren die Abtrennung vor dessen Rückreaktion mit dem Natriumdichromat erfolgt.
• 7. Die resultierende, vom Natriumbicarbonat abgetrenn­te Natriumchromat-/Natriumdichromat-Lösung wird nach Entnahme eines Teilstroms für die pH-Einstel­lung in Schritt 1 und gegebenenfalls Entnahme eines zweiten Teilstroms für die Herstellung von Natrium­dichromat und gegebenenfalls Zusatz von Wasser nun­mehr dem Anodenraum einer Zweikammerzelle mit ka­tionselektiver Membran als Trennwand zugeführt und bei 50 bis 90°C einer Elektrolyse derart unterwor­fen, daß eine im wesentlichen Natriumdichromat ent­haltende Lösung entsteht, die zwecks Ausfällung des darin gelöst enthaltenen Natriumsulfats auf tiefe Temperaturen gebracht werden kann.
• 8. Die im wesentlichen Natriumdichromat enthaltende Lösung aus Schritt 7 wird anschließend einer mehr­stufigen, vorzugsweise 6- bis 15-stufigen Elektro­lyse bei 50 bis 90°C in Zweikammer-Elektrolysezel­len mit kationenselektiver Membran als Trennwand unterworfen, indem die genannte Lösung in die Anodenkammer der ersten Stufe eingeführt wird und nach einer teilweisen Umwandlung des Dichromats in Chromsäure sodann der zweiten Stufe, die eine teil­weise weitere Umwandlung in Chromsäure bewirkt, zu­fließt und so weiter stufenweise bis zur letzten Stufe geführt wird, in welcher ein Umwandlungsgrad des Dichromats in Chromsäure von 55 bis 70 % ent­sprechend einem molaren Verhältnis von Natriumionen zu Chromsäure von 0,45:0,55 bis 0,30:0,70 erreicht ist, wobei die Zahl der Stufen beliebig groß ge­wählt werden kann.
• 9. Diese Chromsäure und einen Rest Natriumdichromat enthaltende Lösung wird durch Eindampfung auf einen Wassergehalt von ca. 12 bis 15 Gew.-% Wasser bei Temperaturen zwischen 55°C und 110°C gebracht, wo­bei der überwiegende Teil der Chromsäure auskri­stallisiert.
• 10. Die erhaltene Suspension wird durch Zentrifugieren bei 50 bis 110°C in einen im wesentlichen aus kri­stalliner Chromsäure bestehenden Feststoff und in eine flüssige Phase, Mutterlauge genannt, welche das gelöst gebliebene Natriumdichromat und die nicht auskristallisierten Anteile Chromsäure ent­hält, aufgetrennt.
• 11. Die erhaltene Mutterlauge wird ständig oder perio­disch oder in unregelmäßigen Abständen derart ge­teilt, daß der weit überwiegende Teil oder zeitwei­se auch die gesamte Menge, gegebenenfalls nach Ver­dünnen mit Wasser, in die Elektrolyse an eine ge­ eignete Stelle, d.h. in eine Stufe möglichst ähnli­chen Dichromat-Umwandlungsgrades, zurückgeführt und ein kleinerer Anteil der Mutterlauge den in Schritt 7. genannten, nicht der Chromsäure-Erzeugung die­nenden, Natriumchromat und Natriumdichromat neben­einander enthaltenden Lösungen hinzugefügt, um so­mit einerseits Verunreinigungen aus dem Elektroly­se-Kreislauf auszuschleusen und andererseits in den genannten Natriumchromat/Natriumdichromat-Lösungen die Restabsäuerung zum Natriumdichromat zu bewir­ken.
• 12. Der in Schritt 10. erhaltene Feststoff wird durch einmaliges oder mehrmaliges Waschen mit 10 bis 50 Gew.-%, bezogen auf Gewicht des Feststoffs, ge­sättigter oder nahezu gesättigter Chromsäure-Lö­sung, die extern oder in situ mit Wasser herge­stellt wird, bei Temperaturen über 35°C und durch jeweils an jedem Waschvorgang anschließendes Zen­trifugieren von anhaftender Mutterlauge befreit.
• 13. Die dabei angefallene Waschflüssigkeit wird in die unter 9. aufgeführte Eindampfung zurückgeführt, wobei eine Nutzung der bei mehrmaligem Waschen des Feststoffes fraktionsweise anfallenden Waschflüs­sigkeiten als Waschlösung im nächsten Zentrifugen-­Zyklus in der Weise möglich ist, daß nur jeweils der letzte Waschvorgang/die letzten Waschvorgänge mit reiner Chromsäure-Lösung ausgeführt wird/wer­den.
• 14. Die in Schritt 12. erzeugte, reine, kristalline Chromsäure wird danach entweder bei 130°C bis 190°C mit indirekter Beheizung oder direkt mit erhitzten, von Reduktionsmitteln freien und an Wasserdampf un­tersättigten Gasen bei 130°C bis 190°C getrocknet oder aber ohne weitere Behandlung ihrer Verwendung zugeführt oder zu Chromsäure-Lösung verarbeitet.
• 15. Die bei der Elektrolyse anfallenden Gase Sauerstoff und Wasserstoff werden jedes für sich gesammelt und gegebenenfalls gereinigt und einer Verbrennung oder einem anderen Einsatzzweck zugeführt.
• 16. Die bei der Elektrolyse von Chromat/Dichromat-Lö­sung in Schritt 7. im Kathodenraum entstehende Na­triumionen enthaltende Lösung und die in allen Elektrolysestufen in Schritt 8. im Kathodenraum entstehende Natriumionen enthaltende Lösung werden zusammengeführt und gegebenenfalls unter Nutzung der in Schritt 7. und 8. entwickelten und abzu­führenden Elektrolysewärme aufkonzentriert.

In a preferred embodiment, the method is carried out as follows:

• 1. By lining the clinker brick produced using low-sulfur fuels and leaving the chrome ore digestion furnace with water and adjusting the pH to 7-9.5 with dichromate solution and / or another mineral acid, a sodium chromate solution with a sodium chromate is obtained -Content of about 300 to 500 g / l, which may be by precipitation at pH 10 to 13 is freed from co-dissolved vanadate in a known manner.
• 2. This solution is then at 20 ° C to 110 ° C, preferably 60 to 90 ° C, to pH values of 8 to 12, preferably 9 to 11, by adding sodium hydroxide and carbon dioxide or sodium carbonate in a corresponding amount approx. 0.03 to 0.1 mol / l carbonate, in order to precipitate the polyvalent cations as poorly soluble carbonates and / or hydroxides, the precipitation also being able to be carried out in several stages with increasing levels of sodium chromate. A sodium chromate solution freed from polyvalent cations except for less than 5 mg / l taken together is obtained.
• 3. If desired, the content of polyvalent cations is then reduced further in the solution produced under 2. by so-called selective cation exchangers.
• 4. The solution produced after step 2 and 3 if necessary is then concentrated to a content of 750 to 1000 g / l of Na₂CrO₄ by single or multi-stage evaporation.
• 5. In this concentrated solution by a single-stage or multi-stage supply of carbon dioxide up to a final pressure of 4 to 15 bar at a final temperature not above 50 ° C, a pH value below 6.5 adjusted and in this way, with the precipitation of sodium bicarbonate, an at least 80% conversion of sodium chromate into sodium dichromate is achieved.
• 6. The sodium bicarbonate is separated off from this suspension either under continuing carbon dioxide pressure or after the pressure has been let down, the latter method being separated off before its back reaction with the sodium dichromate.
• 7. The resulting sodium chromate / sodium dichromate solution, separated from the sodium bicarbonate, is now taken from the anode compartment of a two-chamber cell with cation-selective after removal of a partial stream for the pH adjustment in step 1 and optionally removal of a second partial stream for the production of sodium dichromate and optionally addition of water Membrane supplied as a partition and subjected to electrolysis at 50 to 90 ° C such that a solution essentially containing sodium dichromate is formed which can be brought to low temperatures for the purpose of precipitation of the sodium sulfate contained therein.
• 8. The solution from step 7, which essentially contains sodium dichromate, is then subjected to a multi-stage, preferably 6- to 15-stage electrolysis at 50 to 90 ° C. in two-chamber electrolysis cells with a cation-selective membrane as a partition by submitting the solution mentioned into the Anode chamber of the first stage is introduced and after a partial conversion of the dichromate to chromic acid then flows to the second stage, which causes a partial further conversion to chromic acid, and so on is gradually led to the last stage, in which a degree of conversion of the dichromate to chromic acid of 55 to 70% corresponding to a molar ratio of sodium ions to chromic acid of 0.45: 0.55 to 0.30: 0.70 is reached, the number of stages being arbitrarily large.
• 9. This solution containing chromic acid and a remainder of sodium dichromate is brought to a water content of approx. 12 to 15% by weight of water at temperatures between 55 ° C. and 110 ° C., the majority of the chromic acid crystallizing out.
• 10. The suspension obtained is separated by centrifugation at 50 to 110 ° C. into a solid consisting essentially of crystalline chromic acid and into a liquid phase, called mother liquor, which contains the dissolved sodium dichromate and the non-crystallized portions of chromic acid.
• 11. The mother liquor obtained is divided continuously or periodically or at irregular intervals such that the vast majority or at times also the entire amount, if necessary after dilution with water, in the electrolysis to a ge suitable place, ie returned to a level of dichromate conversion that is as similar as possible, and a smaller proportion of the mother liquor is added to the sodium chromate and sodium dichromate solutions mentioned in step 7, which do not serve for the generation of chromic acid, in order to prevent contamination from the electrolysis To discharge the circuit and, on the other hand, to effect the residual acidification to the sodium dichromate in the sodium chromate / sodium dichromate solutions mentioned.
• 12. The solid obtained in step 10 is washed once or several times with 10 to 50% by weight, based on the weight of the solid, of saturated or almost saturated chromic acid solution, which is prepared externally or in situ with water, at temperatures above 35 ° C and freed from adhering mother liquor by centrifuging after each washing process.
• 13. The resulting washing liquid is returned to the evaporation listed under 9. It is possible to use the washing liquids obtained in fractions when washing the solid several times as a washing solution in the next centrifuge cycle in such a way that only the last washing process / the last Washing operations are carried out with pure chromic acid solution.
• 14. The pure, crystalline chromic acid produced in step 12 is then dried either at 130 ° C. to 190 ° C. with indirect heating or directly with heated gases free of reducing agents and undersaturated with water vapor at 130 ° C. to 190 ° C. or but fed into their use without further treatment or processed into chromic acid solution.
• 15. The gases resulting from the electrolysis, oxygen and hydrogen, are each collected individually and, if necessary, cleaned and sent for combustion or another purpose.
• 16. The solution containing sodium ions formed in the cathode compartment in step 7 in the electrolysis of chromate / dichromate solution and the solution containing sodium ions formed in the cathode compartment in step 8 in all electrolysis steps are combined and, if necessary, using the steps 7 and 8 developed and dissipated electrolysis heat concentrated.

For the production of the alkali metal chromates, alkali metal dichromates and therefrom the chromic acid, technically only chromium iron is used, which is mixed first with sodium carbonate or sodium carbonate / sodium hydroxide solution or sodium hydroxide solution, occasionally with the addition of alkaline earth metal oxides and / or carbonates, in particular calcium oxide and / or calcium carbonate, as an alkaline melting medium and secondly in a mixture with a lean agent consisting essentially of iron (III) oxide or hydroxide, preferably so-called re-ore from the leaching stage described below, is exposed at temperatures above 1000 ° C. to the action of oxygen-containing gases, preferably air.

The comminuted material is leached out, usually in countercurrent, over several stages in order to obtain sodium chromate as a solution with a Na₂CrO₄ content of approx. 300 to 500 g / l. A pH value of 7.0 to 9.5 is necessary to keep the sodium chromate solution as low as possible. This pH adjustment can take place during leaching or in the solution after separation from the leached solid. In order not to introduce new impurities into the system, the required pH adjustment is carried out with dichromate or with chromic acid or with chromic acid / sodium dichromate mixtures or with sodium chromate / sodium dichromate solutions, preferably with those which are used later in the process after acidification Carbon dioxide occur under pressure or with mixtures of said, preferably used sodium chromate / sodium dichromate solutions with sodium dichromate-chromic acid solutions, the latter being removed from the chromic acid electrolysis crystallization cycle for the purpose of discharging impurities. After the pH has been adjusted, the undissolved, leached portions of the ground kiln goods and, if the pH is adjusted only after the leaching residue has been removed, the contaminants that have failed during the pH adjustment Sodium chromate solution is filtered off or centrifuged or separated by allowing the solids to settle. The leached out residue of the kiln material is partly returned to the digestion of the chrome iron stone as a so-called re-ore.

The sodium chromate solution freed from the impurities precipitable at pH 7.0 to 9.5, unless the digestion of the chromium ore has been carried out in such a way that vanadium cannot dissolve during leaching, is now in the form of calcium in the form known per se of calcium oxide or calcium hydroxide in aqueous solution or slurry added to precipitate the vanadium as calcium vanadate. The calcium is used in a stoichiometric excess, taking into account the calcium dissolved in the leaching of the kiln clinker.

After removal of the calcium vanadate, the remaining sodium chromate solution is brought to 50 to 100.degree. C., preferably 70 to 85.degree. C., to precipitate the polyvalent ions remaining in solution despite the pH adjustment, in particular the excess calcium ions used, and with sodium hydroxide solution and carbon dioxide and / or sodium carbonate and / or sodium bicarbonate adjusted to pH 8 to 12, preferably 9.0 to 11.0. The addition of carbon dioxide and / or sodium bicarbonate and / or sodium carbonate is carried out in an amount which has a concentration of carbonate ions in the amount of 0.01 to 0.18 mol / l, preferably 0.03 to 0.1 mol / l in the solution generated. The precipitation can also take place in several stages increasing amounts of sodium chromate are carried out. During a ripening and dwell time of 5 to 360 min while maintaining the pH, the precipitation of the calcium, the strontium and other polyvalent ions and surprisingly also the fluoride takes place, so that after the precipitation has been separated off, a sodium chromate solution with extremely low residual contents is present Contamination results. In the sodium chromate solution produced in this way, residual calcium and strontium contents of less than 5 mg / l are present, while other polyvalent cations such as barium, magnesium, iron, zinc, etc. and also fluoride ions are no longer present or only in an amount below that the respective detection limit, the detection limits being between 0.5 and 1 mg / l. The precipitate filtered off surprisingly contains the precipitated cations practically exclusively as carbonates and as hydroxides, only to a very minor extent as fluorides and chromates, although the latter are clearly in the majority in solution compared to carbonate and hydroxide ions.

wenngleich die Pulverform oder die Gelform für das Ein­rührverfahren ebenfalls wirksam ist. Es ist vorteilhaft, solche Perlpolymere einzusetzen, bei denen die H-Ionen der Säuregruppen in den Substituenten durch Natriumionen ersetzt sind.It has now been shown that for the later, ie downstream, step of electrolysis of sodium dichromate / sodium chromate solution to sodium dichromate and further to chromic acid-containing solution, it is advantageous to further reduce the polyvalent cation content to values below 1 mg / l for each multivalent cation still in solution. This aim is achieved according to the invention in such a way that the sodium chromate solution obtained after the preceding steps has a so-called selective cation Exchanger is given, which consists of macroporous bead polymers based on crosslinked polystyrene with chelating groups, the chelating groups being substituents from the series

although the powder or gel form is also effective for the stir-in process. It is advantageous to use pearl polymers in which the H ions of the acid groups in the substituents have been replaced by sodium ions.

The exchanger is to be regenerated by treatment with acid and is to be freed from the residues of the foreign anions introduced with the regenerating acid by washing with pure water and then to be converted into the sodium form with sodium hydroxide solution, so that the selective cation exchanger is then ready for use again. The various techniques for loading cation exchangers with the cations to be removed from solutions, for connecting and operating different exchange units in series or in parallel, and preferably for alternating regeneration, are known from the literature. The working temperature for the removal of the multivalent cations from the sodium chromate solution is 20 to 90 ° C, preferably wise 60 ° C to 85 ° C, the contact time of solution and exchanger is at least 2 min, preferably 6 min and more.

Before further treatment of the sodium chromate solution, it is advantageously evaporated further down to contents of 750 g / l to 1000 g / l Na₂CrO₄.

The conversion of sodium chromate into sodium dichromate takes place in the process according to the invention with carbon dioxide. This so-called acidification of the sodium chromate can be carried out in one or more stages, and the first stage / the first stages can be operated without pressure; For the desired end result of an at least 80% conversion of sodium chromate into sodium dichromate, however, a carbon dioxide pressure of 4 to 15 bar, preferably 8 to 15 bar, at a final temperature below 50 ° C. is required in the last stage or in the last stages , preferably below 30 ° C. An at least 90% conversion of the sodium chromate at a pressure of more than 8 bar is preferred. In the case of a multi-stage procedure, it is advantageous to increase the carbon dioxide pressure from stage to stage and to combine the material flow of the liquid phase with a separation of the precipitated sodium bicarbonate after each stage, for example by centrifuging under pressure. On the other hand, there is also the possibility of quickly separating the precipitated sodium bicarbonate after relieving the pressure by filtering, centrifuging or decanting. Due to the possible back reaction of sodium dichromate and sodium bicarbonate, there is a close temporal relationship the consequences of relaxation and phase separation are of crucial importance. At the same time as the partial conversion of the sodium chromate to sodium dichromate, the mixture of sodium hydroxide solution and sodium carbonate from the preceding stages present in the sodium chromate solution takes place in sodium bicarbonate.

The sodium bicarbonate obtained here can be converted into sodium carbonate, which is useful for chromium ore digestion, by thermal treatment and / or reaction with sodium hydroxide solution.

A partial stream for the electrochemical generation of chromic acid is removed from the solution now present, which contains at least 80%, preferably at least 90%, of the chromium (VI) as dichromate and practically no longer contains polyvalent cations in detectable amounts. Another partial flow is used for the previously described pH adjustment during / after the leaching of the kiln goods. If desired, further parts of the solution of the preparation of sodium dichromate by adding sulfuric acid or by adding chromic acid or by adding chromic acid-sodium dichromate or by electrochemical acidification, as described, for example, in US Pat. No. 3,305,463 or as below for described the partial stream used for the production of chromic acid, which measures can also be taken simultaneously. For example, the combination of electrochemical acidification with a simultaneous addition of dichromate-chromic acid solution in batches or continuously is a suitable method for complete conversion of the remaining sodium chromate to sodium dichromate in the non-chromic acid partial stream.

To generate chromic acid, the partial stream provided for this purpose is introduced into the anode chamber of a two-chamber electrolysis cell, the partition between the anode and cathode spaces of which is a cation-selective membrane, and therein into a solution which essentially contains sodium dichromate and only minor amounts of sodium chromate and / or chromic acid , electrolytically converted. As a rule, a larger number of such electrolysis cells, e.g. can be summarized in the manner of filter presses, operated in parallel. The voltage required to achieve a current density of 1 to 5 kA / m², preferably 2.5 to 3.0 kA / m², can either be applied individually to each cell which is electrically insulated from the other or, if the cells are conductively connected to one another, in a so-called bipolar circuit at the ends of such an electrically connected arrangement. The voltage to be applied is a function of the electrode spacing and the electrode construction, the solution temperature, the solution concentration and the current strength and is 3.8 to 6.0 V per electrolysis cell.

Each electrolytic cell has in the anode compartment a feed line for the sodium chromate / sodium dichromate solution to be used and a drain for the electrolyzed solution essentially containing sodium dichromate. Supply and drain are usually against set ends of the respective electrolysis cell, the feed advantageously being in the lower part of the electrolysis cell and the outflow in the upper part of the electrolysis cell. In the same way, the cathode chambers are equipped with inlet and outlet. Liquid is pumped both from the anode chamber and from the cathode chamber via external heat exchangers via separate openings in the frame of the cell or, as preferred, via the same openings as for the inlet and outlet. The streams to be pumped from the entirety of the anode chambers and the cathode chambers are advantageously combined to form an anolyte stream and a catholyte stream and are conducted via an anolyte cooler and a catholyte cooler. From these coolers, the cooled anolyte and catholyte liquids are again distributed to the individual anode and cathode chambers. By means of this cooling, the temperature in the anode compartment and cathode compartment is kept at 50 ° C. to 90 ° C., preferably 70 to 80 ° C.

The electrolysis products oxygen and hydrogen are led out of anode spaces or cathode spaces via their own openings in the frame in the upper part of the cell and at the same time or exclusively via the same opening as the processes. The gas streams are advantageously brought together separately after the gases and, if appropriate, freed of entrained solutions and then used, for example, as heating and fuel material in the chrome ore digestion furnace.

Water is entered into the cathode compartments either directly via the feed pipe or by adding to the catholyte liquid in the cooling circuit e.g. after catholyte cooler.

So much solution is always removed from the anode compartments, for example regulated by an overflow, that the molar amount of chromium (VI) taken out in one time period is equal to the amount of chromium (VI) added in the same time period as the sum of sodium chromate, sodium dichromate and chromic acid. than the sum of sodium chromate and sodium dichromate. The cathode compartment liquid of the desired concentration is removed from the cathode compartments, for example regulated by an overflow and controlled by the amount of water introduced into the cathode compartments. The cathode liquid usually consists of 8 to 30%, preferably about 12 to 20% sodium hydroxide solution; The cathode compartment liquid can be modified if desired by adding agents which dull the alkali produced, for example carbon dioxide and / or sodium dichromate solution and / or sodium dichromate / sodium chromate solution from the aforementioned acidification with carbon dioxide. During the ongoing operation of the cells, as much alkali is withdrawn per unit of time as is produced in this period by transporting the sodium from the anode compartments through the membranes into the cathode compartments in the cathode compartments. The concentration of the cathode compartment liquid can be adjusted via the water supply, it is preferably chosen to be as high as possible, restrictions mainly result from the membrane material used.

Cation-selective membranes, which can be used as partitions between the anode and cathode space in the two-room electrolysis cells in the process according to the invention, have already been described several times and have been on the market for a long time. Reinforced membranes with increased durability are preferred by incorporating fibers and fabrics. Both single-layer membranes and bimembranes, consisting of two different membrane types on top of one another, can be used, the two-layer membranes opposing the possible diffusion of hydroxide ions through the membrane with a higher resistance, thus offering the advantage of a higher current efficiency. The suitable membranes have a perfluorocarbon polymer structure with sulfonate exchange groups, suitable reinforcing materials are also fluorocarbon polymers, preferably polytetrafluoroethylene, they are commercially available, for example, under the names ^® Nafion 324, Nafion 435, Nafion 430 and Nafion 423, from DuPont, UNITED STATES.

The electrodes to be used on the cathode side are those that have already proven themselves in chlor-alkali electrolysis in the production of sodium hydroxide solution of various concentrations. They are usually made of steel, stainless steel or nickel and can be activated to reduce the overvoltage of the hydrogen.

The electrodes to be used on the anode side must be resistant to the attack of the acidic and oxidizing medium and to the electrolytically generated oxygen. They consist of a titanium framework and are optionally coated with platinum or with platinum / iridium with a predominant iridium content after application of an oxide intermediate layer made of titanium oxide or tantalum oxide or tin oxide either by wet electroplating or melt electroplating or by the baking process. The types of anodes that can be used are those that have proven themselves in other gas-generating processes, for example anodes in perforated plate shapes, expanded metal anodes, knife anodes, spaghetti anodes and blind anodes. The distance between the electrodes is chosen to be as small as possible, preferably less than 10 mm.

Materials that are resistant to sodium dichromate, in particular titanium and post-chlorinated PVC, are suitable as construction material for the electrolysis cells.

The thus produced, very pure solution, essentially containing sodium dichromate and containing only minor amounts of sodium chromate or chromic acid, is now wholly or partly fed to a multi-stage electrolysis by introducing the above-mentioned solution into the anode chambers of the first stage and partly there in chromic acid is converted and then introduced into the anode chambers of the second stage, where it is converted by a further portion into chromic acid and then continues through the third, fourth and further stages to the last stage. The degrees of conversion of sodium dichromate to chromic acid in the individual stages are such that in the last stage a conversion to 55 to 70%, preferably 59 to 65%, has taken place, so that a Sodium ion: chromic acid ratio of 0.45: 0.55 to 0.30: 0.70, preferably from 0.41: 0.59 to 0.35: 0.65.

The electrolysis cells used for this conversion in all stages are similar to those described in the last section for converting the sodium chromate / sodium dichromate solution into a solution essentially containing sodium dichromate, and they are preferably set up and operated together with them , so that their current and voltage supply as well as their hydrogen and oxygen cleaning and disposal as well as their cathode room liquid treatment, their cooling and concentration and disposal can be summarized. In particular, the same monopolar or bipolar current and voltage supply is selected, the current density here too is 1 to 5 kA / m², preferably 2.5 to 3.0 kA / m², and the voltage to be applied per electrolysis cell is 3.8 to 6.0 Volt; higher voltages are possible, but are avoided for economic as well as technical reasons. The product of the respective previous stage is fed to the electrolytic cells via the feed line of the anode chambers, and the product is fed to the next stage in each case via the outlet. The anolytes are collected for each stage and passed over a heat exchanger for the purpose of heat dissipation and fed back cooled on the opposite side of the anode chamber in the lower part. The total number of heat exchangers for anolytes is therefore equal to the number of electrolysis stages. The catholytes can be combined for all stages and are then preferred together wise combined with the cathode liquid from the step described above of converting sodium chromate / sodium dichromate into sodium dichromate solution, cooled and then redistributed to the individual cathode compartments. Corresponding to the supply of water into the cathode compartments or into the cooled cathode compartment fluid to be distributed over the cathode compartments, cathode compartment fluid is removed from the circuit and fed to further processing, for example by concentration. A preferred form of further processing is evaporation in a vacuum in one to three evaporator stages using the heat released during the electrolysis, so that at least some of the heat exchangers with which the electrolytic heat is removed from the catholyte liquid are identical to some of the heat exchangers for the evaporation of the removed cathode compartment liquid. The composition of the cathode compartment liquid is the same as that of the previous step of converting sodium chromate / sodium dichromate solution to sodium dichromate solution. The temperatures of the solutions in the electrolysis cells are 50 to 90 ° C., preferably 70 to 80 ° C. in all stages. Membranes to be used, anodes, cathodes and construction materials are the same as described above.

In order to achieve an even load of all electrolysis cells involved in the process and their components such as membranes, electrodes and frames through the media treated therein, the function of the cells can be changed at certain time intervals that they realize another sodium dichromate-chromic acid conversion step by changing the flow direction of the anode chamber liquids. By totally reversing the direction of flow of the anolyte, the electrolysis stage with the highest degree of conversion to chromic acid can take over the function of the stage with the lowest degree of conversion and vice versa.

By partially changing the flow direction of the anode chamber liquids instead of completely reversing the flow direction, each cell arrangement can thus take over the function of each electrolysis stage in chronological order.

The anode chamber liquid removed from the last stage of the multi-stage electrolysis is fed to a one to three stage evaporation, the last evaporation stage being designed as an evaporation crystallizer. It is evaporated to such an extent that a crystallization of chromic acid takes place when the solubility limit is exceeded. It is preferably evaporated to a water content in the mixture of 9 to 20% by weight, particularly preferably to a water content of 12 to 15% by weight. The temperature to be set in the crystallizer is 50 to 110 ° C., preferably 55 to 80 ° C., particularly preferably approx. 60 ° C. Various types of crystallizers or crystallization evaporators with an internal heating chamber or with an external heating circuit are suitable for the preferably continuous crystallization. In any case, they must be operated at reduced pressure, so that the evaporation tion can be carried out at the above temperatures. Crystallizers made of titanium are preferably used, which make it possible to produce crystals free of fine grains, that is to say those in whose operation the crystal suspension is screened at least partially according to the crystal size; these are so-called FC (forced circulation) crystallizers and also guide tube crystallizers, for example in combination with hydrocyclones or settling tanks / tanks; Guide tube crystallizers with a clarification zone are even more suitable, e.g. double propeller (DP) crystallizers and fluidized bed crystallizers (see W. Woehlk, G. Hofmann, International Chem. Engineering 27 , 197 (1987); RC Bennett, Chemical Engineering 1988, p. 119 ff).

The crystal sludge removed from the crystallizer can be further thickened via a liquid cyclone (hydrocyclone) or settling container and is either placed directly or after thickening on a centrifuge which is made of titanium in its parts in contact with liquids. The liquid is spun off the crystal cake as far as possible and then the crystal cake is washed one or more times, preferably one to three times, with saturated or almost saturated chromic acid solution. The saturated or almost saturated chromic acid solution can be obtained outside the centrifuge by dissolving chromic acid, preferably by dissolving a portion of the purified chromic acid in the form of the moist, washed filter cake and / or by dissolving an intended fraction of fine particles from the crystalline product dried in the last process step Chromic acid can be produced, but can also be generated in the centrifuge itself by spraying or spraying water or dilute chromic acid solution onto the filter cake. The total amount of water to be used for the washing process is between 3 and 25% by weight, based on the moist centrifuge cake (filter cake), preferably between 4 and 10% by weight, this amount of water is used as such or in the form of a chromic acid solution either all at once or added in portions to the filter cake to be washed out. When washing solution is added in several portions, the resulting solutions running out of the filter cake can be combined or collected separately; in the case of separate pick-up, it is possible to use the processes which are contaminated differently and decreasing from washing step to washing step again in the next centrifuging cycle as washing solution for the respective preceding washing stages. The sequence from the first washing step after centrifuging off the mother liquor or, in the case of single-stage cake washing, the entire washing liquid running off is fed to the evaporation crystallizer, the temperature of the solution being maintained or increased along the way.

The mother liquor of the chromic acid crystallization flowing out of the centrifuge, which is saturated or slightly supersaturated in chromic acid, is mostly fed to the anode side of the multi-stage electrolysis of sodium dichromate to chromic acid without further cooling. It is a conversion for the supply of mother liquor, the composition of sodium dichromate and chromic acid degree of sodium dichromate in chromic acid of approx. 50% corresponds to the level selected among the electrolysis levels which most closely corresponds to the degree of conversion according to the incoming mother liquor. The applicable electrolysis level can be determined mathematically and / or experimentally. Provided that all electrolysis stages have the same or approximately the same electrode and membrane surfaces and are operated with the same current density - as is preferably the case in a layer - the fourth electrolysis stage is suitable for the mother liquor in an eight-stage system, however, in an eleven-stage electrolysis system it is fifth electrolysis stage. To increase the conductivity, water can be added to the mother liquor before it enters electrolysis, or the corresponding amount of water can be fed directly into the anode chambers or the associated cooling circuit for the anode liquid. Any amount of water added is limited so that the water content of the resulting solution does not exceed 50% by weight, that is to say between 25 and 50% by weight.

A small part of the mother liquor flowing out of the centrifuge is fed into the upstream acidification stages for the removal of impurities which have been introduced into the electrolysis circuit, i.e. either into the partial stream removed in process step 7 for the pH adjustment in step 1 or else, as preferred, in the partial stream withdrawn in step 7 for the production of sodium dichromate. In the former case, the discharged solution runs through again all the cleaning steps listed to remove accumulated impurities, in the second case the discharged solution completely leaves the chromic acid production process. If one speaks of the smaller part of the mother liquor flowing out of the centrifuge, then the smaller part in the long-term average is referred to. In the short term, it is not necessary to remove impurities in this way, since only very small, practically hardly measurable amounts of impurities are introduced into the electrolysis system with the sodium chromate / sodium dichromate solution in step 7. Likewise, if it is necessary for economic reasons, a very high proportion of the mother liquor can be removed for a limited period of time in order to serve as a means of pH regulation and chromate-dichromate conversion due to its high acid content. In this context, short-term is considered to be a period of time that does not exceed approximately thirty times the period in which the average volume of sodium dichromate solution flowing from step 7 of the multi-stage electrolysis is the total anode liquid volume of the multi-stage electrolysis including cooling circuits and the crystallizer and the stack container possibly installed in this anode liquid flow. Preference over the unevenly timed removal of mother liquor, however, is the timely even removal of a small part of the mother liquor into the streams of sodium dichromate solution, which are used for producing sodium dichromate or for adjusting the pH in step 1.

A small part of the mother liquor is to be understood as meaning a part which contains between 2% and 20%, preferably between 5% and 10%, of the molar amount of chromium (VI) which is introduced from step 7 into the multi-stage electrolysis.

The pure, crystalline, moisture-laden chromic acid generated in step 12 can be converted into salable goods in different ways after removal or ejection from the centrifuge. If a chromic acid solution generated outside the centrifuge is used to wash the chromic acid crystals in step 12, this moist chromic acid crystal cake is well suited for this and a corresponding proportion is removed. A commercially available, very pure chromic acid solution can also be prepared from the moist crystal cake without further treatment. In order to obtain dry, crystalline goods, the water must be removed below the chromic acid decomposition temperature, that is to say in the temperature range below 195 ° C., preferably from 165 to 185 ° C. On the one hand, there is indirect heating with steam or with a circulating liquid, whereby the material to be dried can, if desired, be kept at reduced pressure, or the direct heating with hot gas, which does not contain any components that have a reducing effect below 195 ° C, and water is undersaturated, suitable. Devices in which chromic acid can be dried according to the principles of contact drying or conventional drying are described in Ullmann's encyclopedia of tech African chemistry, 4th edition, volume 2, pp. 698 ff, (in particular pp. 707 to 717), Weinheim 1972. Devices are preferably used which avoid mechanical abrasion of the crystals or keep them very low, ie those in which the chromic acid crystals are not moved at all or only slowly and little, including slowly rotating, externally heated rotating tubes.

The drying can be followed by a dedusting by screening or classifying in order to remove dusty or finely crystalline fractions, wherein the fine material which has been intended can be used to prepare chromic acid solution for washing the thrown off chromic acid crystals in the centrifuge in step 12.

The gases produced during the electrolysis, oxygen in the anode chamber and hydrogen in the cathode chamber, are each drawn off individually from the electrolysis chambers, normally from the upper part of the electrolysis cell and together with the respective anode chamber liquid or cathode chamber liquid. To remove entrained fine droplets of anode chamber or cathode chamber liquid, the gas streams can, for example, be washed with water or passed over so-called droplet separators or mist separators. In particular, in order to safely remove traces of chlorine, which can result from a low content of chloride in the sodium chromate and sodium dichromate solutions used, contacting the oxygen stream with an absorbent reactive towards chlorine, for example aqueous sodium hydroxide solution and moist activated carbon, is recommended.

Unless another use is preferred, both the oxygen and the hydrogen are fed into the chrome ore digestion furnace in separate lines as oxidizing agent or as fuel. However, it is also possible to burn the hydrogen and discharge the oxygen into the atmosphere.

In the electrolysis of sodium chromate / sodium dichromate solution to sodium dichromate solution and in the further multi-stage electrolysis thereof to a chromic acid / sodium dichromate solution, a sodium alkali product is formed in the cathode compartments in addition to hydrogen, from the hydroxide ions generated at the cathode and from Anode spaces immigrated over the cation-selective membranes, as already described above.

To remove dissolved or finely divided hydrogen from the solution removed from the cathode liquid cooling circuit, the same can be treated before further processing, preferably evaporation in vacuo, for example by heating at normal pressure. The sodium alkali product from the cathode compartments is preferably used for the production of solid sodium carbonate for chromium ore digestion and as a conditioning agent for the chromium ore residue and for sodium chromate solution. Intermediate stages on the way to solid sodium carbonate can be: dilute and concentrated sodium hydroxide solution, sodium carbonate solution, sodium bicarbonate.

Claims (9)

1. Process for the preparation of chromic acid by multi-stage electrolysis of dichromate and / or monochromate solutions in two-room electrolytic cells, the anode and cathode spaces of which are separated by cation exchange membranes, at temperatures of 50 to 90 ° C., the dichromate and / or Monochromate solutions can be obtained by digestion of chromium ores and leaching, characterized in that the monochromate solution obtained after leaching, optionally after the removal of aluminum, vanadium and other impurities, at 20 to 110 ° C to a pH of 8 to 12 by addition and / or in situ production of carbonate in an amount of 0.01 to 0.18 mol / l (at 300 to 500 g / l Na₂CrO₄), separates the precipitated carbonates or hydroxides, the solution to a content of 750 to Concentrates 1000 g / l Na₂CrO₄, converts it with CO₂ under pressure into a dichromate-containing solution, the dichromate-containing solution into the anode compartment of the Ele introduces ktrolysis cell, electrolytically generates a solution containing chromic acid, in which the molar ratio of Na ions to chromic acid is 0.45: 0.55 to 0.30: 0.70 and the chromic acid is worked up by crystallization, washing and drying. 2. The method according to claim 1, characterized in that the electrolysis temperature is 70 to 80 ° C. 3. The method according to claim 1 or 2, characterized in that the electrolysis is carried out in 6 to 15 stages. 4. The method according to any one of claims 1 to 3, characterized in that the ratio of Na ions to chromic acid is set to 0.4: 0.6. 5. The method according to any one of claims 1 to 4, characterized in that the starting solution (monochromate solution) is treated with a cation exchanger. 6. The method according to any one of claims 1 to 5, characterized in that the electrolysis is carried out at a current density of 1 to 5 kA / m² anode surface. 7. The method according to any one of claims 1 to 6, characterized in that the mother liquor obtained in the workup of the chromic acid in the electrolysis process is wholly or partly recycled. 8. The method according to any one of claims 1 to 7, characterized in that the crystallization is carried out by evaporating water at temperatures of 60 to 80 ° C. 9. The method according to any one of claims 1 to 8, characterized in that a part of the mother liquor obtained in the crystallization of chromic acid is removed from the electrolysis circuit.